text: By definition, internal transmittance is related to optical depth and to absorbance as
  where
  *τ is the optical depth;
  *A is the absorbance.
  The Beer–Lambert law states that, for N attenuating species in the material sample,
  or equivalently that
  where
  *σ is the attenuation cross section of the attenuating species i in the material sample;
  *n is the number density of the attenuating species i in the material sample;
  *ε is the molar attenuation coefficient of the attenuating species i in the material sample;
  *c is the amount concentration of the attenuating species i in the material sample;
  *ℓ is the path length of the beam of light through the material sample.
  Attenuation cross section and molar attenuation coefficient are related by
  and number density and amount concentration by
  where N is the Avogadro constant.
  In case of uniform attenuation, these relations become
  or equivalently
  Cases of non-uniform attenuation occur in atmospheric science applications and radiation shielding theory for instance.